1. Field of the Invention
The invention relates to static relays used for opening and closing electric circuits.
2. Discussion of the Background
There are at present at least two main types of relays depending on their technology of construction, electromechanical relays and semiconductor-based relays or static relays. Relays of the electromechanical type comprise one or more electric contacts coupled mechanically to a movable element of a magnetic circuit actuated by controlling the supply to a coil which produces an induction flux in the magnetic circuit. The magnetic circuit is open or closed depending on the position of its movable element. For example, the relay is in the quiescent position, the coil of the relay is not supplied, the magnetic circuit of the coil of the relay is open and the electric contacts of the relay are open. The change of state of the relay is effected by supplying the coil of the relay with an electric control current, the magnetic circuit closes, causing the closure of the electric contacts. The relay goes from a quiescent position to a working position.
The state of the electromechanical relay can be determined rapidly, either by viewing the air gap, between a fixed part and the movable element of the magnetic circuit, or by the switching noise of the relay after a change of state, assuming that the initial state was known.
Unlike electromechanical relays, static relays do not use movable mechanical elements but semiconductors capable of opening or closing an electric circuit in which they are inserted. Generally, for safety reasons the control circuit is galvanically isolated from the electric circuit in which the relay is inserted, certain electric circuits employing high voltages and currents. For example, in the case of static relays the isolating of the control is carried out by a photocoupler. The static relays wired to an electric circuit operating in A.C. mode use semiconductors such as triacs or thyristors mounted in parallel head-to-tail, or analogous components such as those marketed under the names "Snubberless" or "alternistor" which operate like triacs and are better at withstanding untimely triggerings.
These types of semiconductors exhibit, during normal operation, two different states when they are inserted into an electric circuit. An open state and a closed state. In the open state, all of the voltage of the electric circuit is applied to the terminals of the semiconductor. In the closed (or saturated) state, a saturation voltage of around 1.5 volts A.C. appears at the terminals of the semiconductor.
Static relays may exhibit another additional state which is not a customary operating state and which is related to the destruction of the semiconductor, then either being short-circuited or placed in open circuit. When the semiconductor is destroyed and behaves as short-circuited, the voltage at its terminals is virtually equal to 0 volts and when it behaves as an open circuit, the whole of the voltage of the electric circuit is present at its terminals.
Automatic controllers very commonly use relays and particularly static relays which are tending to replace old-technology electromechanical relays and it is necessary to know the actual state of the relay as a function of the state of its control. Currently, two cases may arise, either the relay provides no information about its state, or its state is determined with the aid of systems external to the relay.
For example, making the assumption that the relay is not defective or faulty, a measurement of the current of the electric circuit in which the relay is inserted makes it possible to determine whether the relay is open or closed, this requiring a measurement device which can carry the entire current of the electric circuit. This measurement device is expensive and bulky in particular in the case of electric circuits involving large power levels. In the case of semiconductor-based static relays, the voltage at the terminals of the relay can be measured so as to determine the state thereof. When the relay is open the whole of the voltage of the electric circuit is applied to its terminals and when it is closed it should be possible to detect a voltage of the order of 1.5 volts corresponding to the saturation voltage of the semiconductor. The device for measuring the voltage at the terminals of the relay must on the one hand carry the voltages of the electric circuits which may sometimes be as high as a few hundred or even a few thousand volts and on the other hand have sufficient accuracy to detect the low saturation voltage of the semiconductor of around 1.5 volts. This leads to a measurement rig which is overly expensive and difficult to implement.